The translation of this reads "they have to have books," but in a previous exercise in this lesson, I was marked wrong for translating "je dois vous dire" as "I have to talk to you." The correct answer was "I must talk to you." So I expected to see "they must have books" here. Nope!
I don't understand the difference. I'm okay with being wrong (happens a lot), but is a distinction at work that I don't see, or is this an inconsistency that perhaps needs reporting?
"ils doivent avoir des livres" can indeed translate to "they must have books".
By the way, "je dois vous dire" = "I have to tell you" or "I must tell you"
I have to talk to you = je dois te/vous parler (+ il faut que je te/vous parle).
Then why is avoir used in other exercises for "getting an apple" if it can only mean "have" and not "get"?
Why isn't "They must have..." correct? It means exactly the same as "They have to have..." in standard usage of English!
Why is "They are supposed to have books" not work here? Is it not the same thing?
Not necessarily, because "devoir" has several meanings.
"They are supposed to have books" exactly back translates to "ils/elles sont censés avoir des livres".
With the verb "devoir", you would need a conditional mood for the same meaning: "ils/elles devraient avoir des livres".
Should one be able to distinguish between "Il doit" and "Ils doivent" or do they sound the same?
In plural, you should hear the V as the last consonant sound.
il doit = dwa
ils doivent = dwaV
Well I'm happy because I tried 'they've got to have some books ' and it was accepted. 'got to' being rather more in use in UK I thought it would flop, but no.